When packaging electronic circuitry inside an integrated circuit (IC) or a sub-assembly, it is desirable to maximize certain parameters such as circuit density, input/output availability, and circuit functionality, while minimizing certain other parameters such as power consumption, package size and pin count.
Towards this end, a 5-pin sub-assembly package has been traditionally used to house an optical device such as an optical receiver or an optical transmitter together with their associated electronic circuitry. One such optical device is an optical receiver package commonly referred to as a receiver optical sub-assembly (ROSA). A 5-pin ROSA incorporates a first pin that is used for connecting to a positive voltage, a second pin for connecting to ground, third and fourth pins for outputting a differential data signal derived from an optical signal received in the ROSA, and a fifth pin that outputs a monitor signal representing the signal strength of the received optical signal. As can be appreciated, this type of predetermined pin assignment places severe limitations on any additional functionality that can be provided by the ROSA. For example, the lack of an input pin prevents a user of the device from feeding a desired control signal into the ROSA.
As a further example, another traditional optical device is an optical transmitter circuit commonly referred to as a transmitter optical sub-assembly (TOSA). A 5-pin TOSA typically incorporates a first pin that is used for connecting to a positive voltage, a second pin for connecting to ground, third and fourth pins for receiving a differential electrical data signal that is converted into an optical signal for transmission out of the TOSA, and a fifth pin that outputs a monitor signal representing the signal strength of the transmitted optical signal. Here again, the predetermined pin structure places undesirable constraints upon a user of the device.